This application relates generally to a device and method for tracking the baseline of a radiation detector. More specifically, this application relates to a method which more accurately corrects a medical imaging gamma detector for baseline drift.
In the field of Medical Imaging, one modality is nuclear medicine (gamma camera) imaging. This imaging is known to use a detector consisting of a scintillator backed by a plurality of either single anode photomultiplier tubes (PMTs) or multi-anode position sensitive PMTs (PSPMTs) with appropriate electronics. For brevity in the upcoming discussion, PMTs and/or PSPMTs will be referred to as PMTs, but anyone skilled in the art will recognize that either can be used with appropriate modifications.
In its application, a patient is given a radioisotope either by injection or ingestion and then the detector(s), after being placed in close proximity to the patient, can determine where the radioisotope goes or has gone. Then, the device is used to detect the radioisotope as it travels through the patient.
The process of detection is when the radioisotope emits a gamma photon in the direction of the detector; it is absorbed by the scintillator. The scintillator emits a flash of light (a scintilla) which is detected by the plurality of PMTs. The PMTs closer to the flash have a higher signal than those further away. By measuring the intensity of the flash at each PMT, then using a centroid type calculation, a fairly accurate estimation of where the flash occurred is possible. The output of the PMT's is an electrical current proportional to the amount of light detected by each PMT. The PMT output current is converted to a voltage and amplified, then integrated to derive the total energy (light) detected by each PMT. The integration of the PMT output is started by detecting the presence of output current from the PMT, this is referred to as the trigger.
In order for the integrated PMT output to accurately represent the energy of the light detected by the PMT, the baseline of the PMT output must be accounted for. Ideally the baseline is zero, however this is rarely the case. The PMT baseline varies from the ideal due to offset voltages in analog electronics and crystal afterglow in high count rate situations. The offset of the baseline can therefore vary depending on such factors as temperature, power supply and count rate. It is therefore desirable to track the current baseline level on a continuous basis. This baseline value is then used to compensate the integrated PMT output result in some fashion, usually by subtracting the baseline error contribution from the integrated results, or by adjusting the actual baseline. If the baseline is not corrected, the effect is to shift the energy measured by the system, i.e., a shift in the energy peak is observed. Because high quality imaging requires a stable energy peak, drifting baselines can cause a general degradation in image quality.
The traditional baseline tracking technique comprised taking a sample of the PMT output channel when the trigger detection circuit determined that there had been no gamma photon interaction for a period of time long enough so that the PMT output should be in its quiescent state, or that the PMT channel baseline being sampled is not effected by the current gamma photon interaction because it is spatially separated from the gamma photon interaction.
This technique is flawed because it relies principally on the trigger detection circuit to determine PMT output inactivity. There is always the possibility that the PMT output is not at its quiescent state due to noise, or low energy gamma photon interactions that are not detected by the trigger detection circuit. In order to circumvent this problem, the prior art used multiple baseline samples that were either averaged or their weighted mean tracked with a histogram to determine the baseline value. The disadvantage of using multiple baseline samples to arrive at a calculated baseline value is the time required to gather the samples and calculate the representative baseline. Because crystal afterglow is a phenomenon that varies in the millisecond time constant range, in order to track crystal afterglow when the gamma photon source is changing rapidly in either intensity or spatial position would require baseline updates on the order of thousands of time a second. Thus, a means of correcting one or more the above identified shortcomings would be useful.